This invention relates generally to sonar systems and more specifically to a sonar system having an improved signal processor and display for reducing the false alarm rate and for displaying information corresponding to additional objects than heretofore detected and displayed.
As is known in the art, sonar systems are used on sea going vessels to detect the presence of objects in the surrounding water environment. For example, it may be desirable to detect the presence of other vessels such as ships or submarines, as well as reefs and other natural obstacles. Often it is simply desirable to distinguish vessels from natural obstacles. In general terms, a sonar system includes a transmitter, a receiver, a signal processor, and a display. The transmitter is used to transmit acoustic energy or signals, and is generally referred to as a projector. The transmitted signal is reflected by objects in the water environment to provide echoes or reflected signals to the receiver. The receiver is generally comprised of a plurality of receiving elements, such as are provided in a hydrophone array. The signals received by each of the plurality of hydrophones are processed by the signal processor for display and interpretation by the sonar system operator.
As is also known in the art, conventional sonar signal processors include a beamformer for providing a plurality of output signals or beams corresponding to the hydrophone array "looking" in multiple directions. The beams are formed by applying the received signals to phase-shifting or time-delay network.
Some conventional sonar signal processors operate on an energy detection principle. That is, the output signals from the beamformer are energy detected by integrating the squared value of the signal to provide a corresponding plurality of signals indicative of the level of the energy associated with the given beam. This type of signal processing may be unsuitable for distinguishing between objects desired to be detected and other "clutter" detections. In other words, the output signals of the energy detector which are displayed indicate only the presence of an object (i.e. a received echo) and in no way assist the sonar system operator in determining the type of object providing the echo.
Other types of sonar signal processors operate on a matched filter or coherent processor principle. With this arrangement, the output signals from the beamformer are compared to the transmitted signal to determine whether such received signals are replicas of the transmitted signal. Thus, this type of processor may be referred to generally as a replica correlator. It is recognized that when the echo is a replica of the transmitted signal, the likelihood exists that the reflecting object is a vessel of some sort, as opposed to a natural obstacle such as a reef or other clutter. Stated differently, the echo from a vessel more closely approximates a point-like reflector whereas reefs for example, tend to spread out the echo due to their relatively large dimensions. It is noted that such time-spread echoes, such as are reflected off a reef, are better suited for detection by an energy detection scheme whereas, the point-like echo reflected off a vessel is better suited for detection by a matched filter. However, even with the use of a matched filter signal processor, the rate with which such detections falsely indicate the presence of vessels (i.e. referred to hereinafter as the false alarm rate) may be unacceptably high since other obstacles may provide echoes which replicate the transmitted signal.
One way known in the art to reduce the false alarm rate is to use both coherent and incoherent signal processors, as described in U.S. Pat. No. 4,831,603 entitled "Simultaneous Coherent and Incoherent Processor for Sonar Signals" issued on May 16, 1989 with inventors John T. Kroenert and Robert B. Delisle, and assigned to the assignee of the subject invention. The patent describes a technique for displaying only those objects detected with the coherent processor as described above when there is also a detection by the incoherent processor, such incoherent processor being referred to as a polarity coincidence correlator. In this way, the incidence of false alarms is reduced. However, the displayed information is only as sensitive as the least sensitive of the processors. Stated differently, the incoherent processor information is used to "validate" the coherent processor detected objects as being of interest for display and thus, the overall display sensitivity is reduced.